I believe the IMC-rating's success story underpins the safety case for the attainable IR
Couldn't agree more (predictably) but the problem is that we are
not going to get an "attainable" IR, using the probably most widely accepted definition of "attainable" in this context, which is the US one:
1 - 1 exam
2 - largely competence based
3 - doable at your old flying school
4 - rolling currency
5 - PPL-level medical
It looks like the current proposals offer #2 but the others (esp. #3) are just as important to getting market penetration.
We are never going to get an FAA IR equivalent because
a) European regulators hate the FAA system because it is being continually rammed up their noses, so while they are content ripping off FAA certification regs and sticking their own document headings on them
they will not go for what will be an FAA-like IR
b) In Europe, the political (airline pilot, NAA, union) landscape is such that it is judged important to have the same IR for private flight as for the ATPL. A failure to maintain this is judged as carrying a risk of private IFR being segregated by airspace class, one day, and that would strip the IR of any real utility value. In the USA this is not an issue because of historical acceptance, the ATPL having a tighter checkride than a CPL/IR, type rating checkrides being done to ATP standards, etc.
This is why the IMCR should be preserved in the UK. It serves a well defined user base. The IR, in whatever form it arrives (if indeed it does arrive) is always going to be aimed at a different market: reasonably current (well funded) aircraft owners.